1. Field of the Invention
The present invention relates to a piezoelectric-transformer inverter for converting DC voltage to AC voltage through use of a piezoelectric transformer.
2. Description of the Related Art
Recently, back lit liquid crystal displays have come into general use as display units for portable information processing apparatus such as laptop personal computers. A fluorescent tube, such as a cold-cathode tube, is used as a light source of such a back light. In order to light the fluorescent tube, a high AC voltage must be applied thereto. Because a battery and an AC adapter are generally used as an input power source of the portable information processing apparatus such as a laptop personal computer, a DC/AC inverter must be used to convert a low DC voltage supplied from the input power source to an AC voltage that is sufficiently high to power the fluorescent tube.
Recently, there has been developed a piezoelectric-transformer inverter which uses a piezoelectric transformer that is smaller than an electromagnetic transformer. In order to use such transformer to convert power for the fluorescent tube, a piezoelectric transformer is sometimes required to meet the following requirements:
(1) it must be operable at a low voltage; specifically, that output by one lithium-ion cell; and PA1 (2) it must be small and thin.
In relation to Requirement (1): In general, a voltage of 700 V (peak-to-peak voltage) or higher is needed in a steady state to light a fluorescent tube such as a cold-cathode tube, although the specific value depends on the length of the fluorescent tube. However, the voltage of one lithium-ion cell is only about 2.5 to 4 V. To compensate for this difference, the inverter must provide a step-up ratio within the range of 175 (=700/4) to 300 (=700/2.5). Since the luminance efficiency (luminance/power consumption of the cold cathode tube) of the cold cathode tube, including peripheral panel parts, is considered to be highest at a frequency of about 50 to 100 kHz, the drive frequency must generally fall within the range of 50 to 100 kHz.
In relation to Requirement (2): A Rosen-type piezoelectric transformer, which is a basic piezoelectric transformer, will be described with reference to FIG. 1. In a Rosen-type piezoelectric transformer 1, primary electrodes 3 are formed in one half of a piezoelectric substrate 2, formed of piezoelectric ceramics. The primary electrodes 3 are formed in the longitudinal direction, on top and bottom main faces of the piezoelectric substrate 2. The piezoelectric substrate 2 is polarized in a direction perpendicular to the surfaces of primary electrodes 3 (in the thicknesswise direction of the piezoelectric substrate 2) in this area. In the other half of the piezoelectric substrate, the piezoelectric substrate 2 is polarized in the longitudinal direction, and a secondary electrode 4 is formed on an end surface adjacent to the second half region. The directions of polarization of the piezoelectric substrate 2 are shown by arrows P (arrows P are also used to indicate directions of polarization in the drawings relating to the description that follows). In the Rosen-type piezoelectric transformer 1, when an AC voltage output from an input power source 6 is applied to the primary electrodes 3 which face each other with the piezoelectric substrate 2 disposed therebetween the applied voltage is converted to a mechanical distortion. This distortion excites a mechanical vibration in the longitudinal direction of the Rosen-type piezoelectric transformer 1, and the mechanical vibration is then converted to an electrical vibration. Thus, a transformer function is realized, so that a stepped-up voltage is applied to a fluorescent tube 5 serving as a load.
FIG. 2 shows possible vibration modes in the longitudinal direction of the Rosen-type piezoelectric transformer 1. Since the opposite ends of the piezoelectric transformer 1 are open ended as shown in (a), the piezoelectric transformer 1 can vibrate in a .lambda./2 (half wavelength) mode, a .lambda. mode, or a 3.lambda./2 mode, which are shown at (b), (c) and (d), respectively, of FIG. 2 and vibrates at a lowest frequency in the .lambda./2 mode. When the vibration frequency of the .lambda./2 mode is represented by fo, the vibration frequencies of the .lambda. mode and the 3.lambda./2 mode are represented by 2fo and 3fo, respectively. In the .lambda./2 mode, the .lambda. mode, and the 3.lambda./2 mode, the length L of the piezoelectric transformer 1 and the wavelength .lambda. satisfy the equations L=.lambda./2, L=.lambda., and L=3.lambda./2, respectively. In other words, when the piezoelectric transformer is driven at a specific frequency (for example, in the frequency range of 50 to 100 kHz), the size of the piezoelectric transformer can be advantageously decreased through employment of the .lambda./2 mode.
However, in practice, .lambda.-mode piezoelectric transformers have conventionally been used more often than have .lambda./2-mode piezoelectric transformers, because .lambda./2-mode piezoelectric transformers have the following drawbacks. First, the step-up ratios (=output voltage/input voltage) of .lambda./2-mode piezoelectric transformers are generally lower than those of .lambda.-mode piezoelectric transformers. In addition, when voltage input to a piezoelectric transformer is not sinusoidal, a .lambda.-mode vibration is also generated as a harmonic of a .lambda./2-mode vibration. As a result, distortion of the output voltage or current increases. In general, in a piezoelectric-transformer inverter used for a back light of a liquid crystal display, current flowing through the fluorescent tube is monitored, and frequency control is performed in order to maintain the peak value of the monitored current at a constant level. However, when the distortion of waveform is large, the root-mean-square value of the current changes even if the peak value of the current is maintained constant. Therefore, the current-controlling performance is poor.
First Conventional Example
In order to solve the above-described problems and to meet the requirements of piezoelectric-transformer inverters, various improved piezoelectric-transformer inverters and improved piezoelectric transformers have been proposed, as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 9-107684, 9-56175, and 9-74236. FIG. 3 shows a piezoelectric-transformer inverter disclosed in Japanese Patent Application Laid-Open No. 9-107684. This piezoelectric-transformer inverter 11 is formed of a piezoelectric transformer 13, a frequency control circuit 14, a step-up circuit (drive circuit) 15, a drive voltage control circuit 16, and a light adjustment circuit 17. The piezoelectric transformer 13 applies a voltage to a fluorescent tube 12. The frequency control circuit 14 detects current that is supplied from the secondary electrode of the piezoelectric transformer 13 to the fluorescent tube 12, and controls the drive frequency of the piezoelectric transformer 13 in order to maintain the detected current at a predetermined level. The step-up circuit 15 divides the drive frequency, generates a drive voltage having a divided frequency, and supplies the generated drive voltage to the primary electrodes of the piezoelectric transformer 13. The drive voltage control circuit 16 controls the drive voltage such that the drive voltage applied to the piezoelectric transformer 13 is maintained at a predetermined constant level even when the input power source voltage V.sub.DD changes. The light adjustment circuit 17 controls averaged load current (tube current) flowing through the fluorescent tube 12, by means of PWM control.
In the piezoelectric-transformer inverter 11 as well, the drive frequency is controlled by the frequency control circuit 14 and the like such that the output current is maintained constant. The step-up circuit 15 is constructed such that push-pull operation (quasi-E-class operation) is realized through use of two transistors 18 and 19 as well as two coils 20 and 21. In the piezoelectric-transformer inverter 11 of such a push-pull operation, as a result of on/off operation of the two transistors 18 and 19 of the step-up circuit 15, a drive voltage applied between the primary electrodes of the piezoelectric transformer 13 has a waveform similar to a sinusoidal waveform, and is amplified to a high voltage.
Further, since the drive voltage control circuit 16 is provided in a stage preceding the step-up circuit 15 in order to adjust the average voltage applied to the step-up circuit 15, the piezoelectric-transformer inverter 11 can cope with a wide range of input voltage. Moreover, since the drive voltage control circuit 16 is operated and stopped intermittently by the light adjustment circuit 17, the output current can be adjusted within a wide range.
In the piezoelectric-transformer inverter 11, since push-pull operation is effected through use of the two transistors 18 and 19 and the two coils 20 and 21, the voltage input to the piezoelectric transformer can be doubled, so that an insufficient step-up ratio of the piezoelectric transformer can be supplemented. However, by itself, the employment of push-pull operation is insufficient for obtaining a required step-up ratio of about 300 times.
Second Conventional Example
FIG. 4 shows a piezoelectric-transformer inverter disclosed in Japanese Patent Application Laid-Open No. 9-56175. This piezoelectric-transformer inverter 31 utilizes an electromagnetic step-up transformer 32. DC voltage supplied from an input power source 35 is converted to AC voltage by means of a tank circuit composed of a capacitor 34 and a primary coil 33 of the electromagnetic step-up transformer 32. The voltage generated in the primary coil 33 is stepped up in order to extract a stepped up voltage from a secondary coil 36 of the electromagnetic step-up transformer 32. The voltage output from the secondary coil 36 is applied between primary electrodes 38 of a piezoelectric transformer 37, so that a further stepped up voltage is output from a secondary electrode 39 of the piezoelectric transformer 37 and is applied to a fluorescent tube 40. Thus, a load current is supplied to the fluorescent tube 40.
However, since the electromagnetic step-up transformer 32 is generally larger in size than coils (inductors), the piezoelectric-transformer inverter 31 utilizing the electromagnetic step-up transformer 32 is not suitable for miniaturization. Further, in the piezoelectric-transformer inverter 31, vibration occurs in the .lambda. mode due to harmonics of the voltage input to the piezoelectric transformer 37, so that the voltage output from the piezoelectric transformer 37 is distorted.
Third Conventional Example
FIG. 5 shows a piezoelectric transformer disclosed in Japanese Patent Application Laid-Open No. 9-74236. This piezoelectric transformer 41 is of a center drive type. Primary electrodes 43 are formed on the top and reverse main faces of the piezoelectric substrate 42 to be located at the central portion thereof. Secondary electrodes 44 are formed on the longitudinally opposite end surfaces of the piezoelectric substrate 42 in order to form electricity generation sections in the longitudinally opposite end portions. In the piezoelectric transformer 41, the piezoelectric substrate 42 is polarized such that the portion thereof at the drive section is polarized in the direction perpendicular to the primary electrodes 43 while two remaining portions at the generation sections are polarized oppositely in the direction parallel to the primary electrodes 43. In this structure, since the two portions at the generation sections are polarized in opposite directions, the piezoelectric transformer 41 has a symmetric structure along the longitudinal direction with respect to the center thereof. Therefore, the charge generated in the piezoelectric substrate 42 is effectively canceled in the .lambda.-mode vibration, thereby suppressing the harmonic components of the output voltage.
However, since the piezoelectric transformer 41 is of a center drive type, the opposite ends (secondary electrodes 44) become high voltage ends, so that other components cannot be mounted in the vicinity of the ends. By contrast, in the case of a Rosen-type piezoelectric transformer, since high voltage appears at only one end of the transformer, other components can be mounted in the vicinity of the low-voltage-side (primary-electrode side). Therefore, the piezoelectric-transformer inverter utilizing the center-drive-type piezoelectric transformer 41 has a drawback in that, compared to the case of a piezoelectric-transformer inverter utilizing a Rosen-type piezoelectric transformer, its dead space increases and thus its installation area increases.